You need to tread very carefully. There is huge potential here for your son to end up in a long-term abusive relationship from which he cannot escape. She is likely to work quite hard to isolate him from all sources of support, including his family and friends and he will probably believe that allowing this is the right thing to do.
I've been where you are now and we only managed to hold onto my 18 year old DD by the skin of our teeth.
Do everything in your power to keep your relationship with your DS intact. Don't criticise her to him. If you want to discuss her behaviour, ask questions about why she would do those things so you get him to think. Just be puzzled, not angry (outwardly at least).
Don't challenge her. She will probably use that as the way to remove you from his life.
My DD was told within the first few weeks of the relationship that she needed to distance herself from us to show she was committed to being with her abuser. She believed him.
Read everything you can about domestic abuse and coercive control. Make yourself an expert in what is happening to your son.
Welcome her into your home whenever possible. If she doesn't feel welcome, she will make him choose.
Try to find opportunities to spend time alone with him. Remind him how much you love him and that you will always be there for him no matter what.
If he says things or makes decisions that hurt you, tell yourself that this is her, acting through him. Once he is out of the relationship, he will look back and wonder why he did those things. Don't allow it to damage your relationship with him because it may be his only lifeline.
The time my DD was in the abusive relationship was horrific. I knew that losing her was a very real danger. I was terrified and felt powerless to help her.
Every second I spent in his company, my instincts were screaming at me to get him out of our lives, my house, etc and I had to squash them down and be delightful to him. It was the hardest thing I ever had to do.
I got my daughter back and she has now recovered. Her abuser should have gone to prison but, predictably, there was not enough evidence of his crimes to prosecute him.
I hope the outcome is good for you too 